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Occasionally, infections can result in a poor stand and reduced yield. The same virus infects other hosts including bean, squash, cucumber, melons, spinach, beets, and pepper. The curly top virus is spread from plant to plant only by the sugar beet leafhopper. THE VECTOR: Beet Leafhoppers are small, slender grayish insects and 1/8 inch long. They overwinter in the warmer regions of Nevada and Arizona where they pick up the virus and then migrate with the help of spring winds into Utah. They do not overwinter in northern Utah because of the severe winters. Leafhoppers pick up the virus only by feeding on a plant already infected with curly top. A few minutes of feeding is enough to contaminate the insect with the curly top virus for the rest of its life. The insect can only transmit the virus by feeding on host plants. As it sucks the juice from the plant it leaves behind virus particles in the leaf veins. Adult sugar beet leafhoppers are generally better transmitters of the virus than the young nymphs.
In addition to the previously mentioned host crops, many weeds also harbor the virus without developing symptoms. The curly top virus cannot be spread in soil, seed, or by human contact. It also does not survive in the soil.
SYMPTOMS: Once the virus is in the plant, it multiplies very rapidly and symptoms begin to appear one to two weeks after infection. First, and often unnoticed, the topmost tiny leaflets become twisted and yellow. The older leaves become thick and leathery and show an upward rolling, revealing pronounced purple veins. Branches appear stiff and erect. The whole plant gradually declines, becoming chlorotic and brittle before dying. Plants infected when young are severely stunted since growth almost stops after infection with curly top. Fruit on infected plants ripen prematurely and are small and leathery.
CONTROL Controlling the beet leafhopper does not reduce the incidence of curly top disease since the insects migrate long distances and do not reproduce or remain in tomato fields. Since the incidence of curly top varies from year to year, it is advisable to plant a few more plants than you require to compensate for potential losses. Double planting or putting two plants in one hole allows for one plant to get infected and the other to continue growth. Even if both plants remain healthy, the yields will not be seriously reduced by crowding.
Growing tomatoes in the shade of a taller plant such as corn also reduces the chances of infection since leafhoppers do not like to feed in the shade, and shaded plants are slightly more resistant to curly top than those grown in the full sun. Rogue out all infected plants as soon as noticed. They do not recover and will compete for nutrients and water with healthy plants. The virus does not affect humans.
There are several resistant varieties which could be used in high risk areas near weedy fields or the foothills. These include Roza, Columbia, Rowpac, and Saladmaster.
